Light Zn Research Articles

Ca–Zn isotope fractionation during fluid–rock interaction in subduction zones and its implication for deep carbon cycle

Subducting slabs transport Earth's surface carbon into the deep mantle, with a portion of this carbon potentially recycled through volcanic eruptions at mid-ocean ridges or ocean islands. Non-traditional stable isotopes such as calcium (Ca) and zinc (Zn) are increasingly used to trace this deep carbon cycle. However, the potential for Ca and Zn isotope fractionation in the subduction zone, critical for the successful application of these tracers, remains insufficiently explored. In this study, we investigate high-pressure carbonated metamafic rocks and enclosed vein systems from a Paleo-oceanic subduction complex (SW Tianshan) to explore the possible fractionation of Ca and Zn isotope during subduction-related metamorphism and fluid–rock interaction. The δ44/40Ca (0.77 ‰–0.90 ‰) and δ66Zn isotope compositions (0.20 ‰–0.39 ‰) of greenschist-, blueschist- and eclogite-facies metabasites are consistent with their protoliths (OIBs and MORBs). We observe no systematic fractionation of either Ca and Zn isotopes across prograde metamorphism and associated dehydration. However, the precipitation of carbonate in surrounding rock along fluid pathways leads to an increase in the δ44/40Ca of residual fluids (>0.90 ‰). More significantly, we find detectable Zn isotope fractionation (ΔA-H = ẟ66Znaltered - ẟ66Znhost of 0.06–0.10 ‰) during fluid-mediated metasomatism of some blueschists/eclogites. Light Zn isotopes are preferentially released into the infiltrating fluid, resulting in an elevated δ66Zn isotope composition in the immediate carbonated eclogites along the vein. The modified slab fluids during migration are likely charactered by heavy δ44/40Ca and light δ66Zn compositions, which may play a crucial role in determining the low δ66Zn values found in some arc lavas. However, the MORB-like δ44/40Ca compositions observed in global arc magmatic rocks suggest that the slab-released Ca (possibly along with some carbon) is probably sequestered and stored along the migration path of the fluid, thereby hindering the slab–arc carbon recycling. Our study emphasizes that intraslab fluid–rock interaction results in similar δ44/40Ca and δ66Zn signature in carbonate-bearing eclogites compared to sedimentary carbonates. Consequently, deeply subducted carbonate-bearing eclogites –not necessarily sedimentary carbonates– might carry light δ44/40Ca and heavy δ66Zn signatures into the deep mantle, potentially influencing the corresponding Ca-Zn isotope compositions of some mantle-derived basalts.

Balancing the oceanic Zn isotope budget: The key role of deep-sea pelagic sediments

Abstract Oxygenated deep-sea pelagic sediments with Fe-Mn–oxide particles represent a key oceanic oxic sink for transition metals in the modern ocean. However, the isotopic composition of authigenic Zn in the pelagic zone remains poorly constrained, which hampers our understanding of the global budget of Zn isotopes. Here, we analyzed the Zn isotopic compositions of two deep-sea pelagic sediment columns collected from the Pacific Ocean. The results show that authigenic Zn in deep-sea sediments is primarily hosted by the Fe-Mn (oxyhydr)oxides. The light Zn isotopic signatures (δ66Zn: −0.02‰ to 0.34‰, n = 42; computed as the per mille deviation of the 66Zn/64Zn ratio from the JMC-Lyon standard) observed in deep-sea sediments are completely different from the previously assumed values of ~1.0‰ based on the Zn isotopic compositions of Fe-Mn crusts and nodules. Based on this observation (Zn flux of deep-sea sediments = 5.3 × 108 mol yr–1), we propose a new, balanced global budget for Zn isotopes.

Multifunctional Roles of Zinc in Cadmium Transport in Soil-Rice Systems: Novel Insights from Stable Isotope Fractionation and Gene Expression.

The effect of Zn on Cd accumulation in rice varies under flooding and drainage conditions, and the underlying mechanism during uptake and transport from the soil to grains remains unclear. Isotope fractionation and gene expression were investigated using pot experiments under distinct water regimes and with Zn addition to gain a deeper understanding of the molecular effects of Zn on Cd uptake and transport in rice. The higher OsHMA2 expression but constitutively lower expression of zinc-regulated, iron-regulated transporter-like protein (ZIP) family genes in roots under the drainage regime than the flooding regime caused the enrichment of nonheavy Zn isotopes in the shoots relative to roots but minimally affected Cd isotopic fractionation. Drainage regime seem to exert a striking effect on the root-to-shoot translocation of Zn rather than Cd, and increased Zn transport via OsHMA2. The changes in expression patterns in response to Zn addition were similar to those observed upon switching from the flooding to drainage regime, except for OsNRAMP1 and OsNRAMP5. However, soil solution-to-rice plants and root-to-shoot fractionation toward light Zn isotopes with Zn addition (Δ66Znriceplant-soilsolution = -0.49 to -0.40‰, Δ66Znshoot-root = -0.36 to -0.27‰) indicated that Zn transport occurred via nonspecific uptake pathways and OsHMA2, respectively. Accordingly, the less pronounced and minimally varied Cd isotope fractionation suggested that OsNRAMP5 and OsHMA2 are crucial for Cd uptake and root-to-shoot transport, respectively, facilitating Cd accumulation in grains. This study demonstrated that a high Zn supply promotes Cd uptake and root-to-shoot transport in rice by sharing distinct pathways, and by utilizing a non-Zn-sensitive pathway with a high affinity for Cd.

Fluid-mediated Cu and Zn isotope fractionation in subduction zones and implications for arc volcanism: Constraints from high pressure veins within eclogites in the Dabie Orogen

High-pressure (HP) veins in eclogites are the products of fluid-rock interaction and provide insight into the composition and evolution of fluids in subduction zones. We here present the Cu and Zn isotope data for different types of HP veins and their host eclogites from Ganghe and Hualiangting in the Dabie Orogen to reveal the behavior of Cu and Zn isotopes during fluid-rock interaction and fluid evolution. The HP veins include omphacite-epidote (Omp-Ep), epidote-quartz (Ep-Qtz), and kyanite-epidote-quartz (Ky-Ep-Qtz) veins. The Omp-Ep veins first crystallized from eclogite-derived, solute-rich fluids with the Ep-Qtz and Ky-Ep-Qtz veins successively crystallizing from the residual fluids after the Omp-Ep vein formation. The early Omp-Ep veins have variably lower δ65Cu (−1.32 to −1.12‰ at Ganghe, −0.70 to −0.66‰ at Hualiangting) but higher δ66Zn (0.36 to 0.38‰ at Ganghe, 0.40 to 0.41‰ at Hualiangting) relative to the host eclogites (δ65Cu: −0.71 vs. 1.84‰, δ66Zn: 0.22 vs. 0.33‰), indicating CuZn isotope fractionation during slab dehydration in subduction zones with the lighter Cu and heavier Zn isotopes preferentially entering the vein-forming fluids from the eclogites. Systematic increase of δ65Cu from the Omp-Ep (−0.70 to −0.66‰) through Ep-Qtz (0.01‰) to Ky-Ep-Qtz veins (0.46 to 0.95‰) can be attributed to isotope fractionation induced by redox changes during the evolution of metamorphic fluids, as revealed by the negative correlations of δ65Cu with redox-sensitive ratios of Fe3+/ΣFe and (Eu/Eu*)N in those veins. Additionally, the higher δ66Zn of the Omp-Ep veins relative to the Ky-Ep-Qtz veins can be explained by equilibrium isotope fractionation between crystallized minerals and evolved metamorphic fluids. Our results thus demonstrate that CuZn isotope fractionation occurred during the evolution of slab-derived metamorphic fluids in subduction zones. Binary mixing calculations show that fluids derived from dehydration of mafic rocks in subducted slabs, represented by the multistage HP veins in the present study, cannot account for the heavier Cu and lighter Zn isotope compositions in most arc magmas than in mid-ocean ridge basalts. This offset can be resolved by addition of forearc serpentinite-derived fluids enriched in heavy Cu and light Zn isotopes into the mantle source of arc magmas.

The REY geochemistry of phosphorites during metamorphism of the Haizhou Group, NW Yangtze Block, China

Rare earth elements (REE including Y—REY) in phosphorites have been used widely as a proxy for seawater chemistry at the time of their formation. However, it is unclear whether metamorphism can lead to REE remobilization and alteration of primary seawater signatures recorded in phosphorites. In this study, we examine similarly aged metamorphosed phosphorite ore deposits from the northeastern margin of the Yangtze Block (Jinping and Xinpu ore sections), with contemporaneously deposited unmetamorphosed equivalents in the southwestern margin of the Yangtze Block (Weng'an ore section). New insights have been provided into the migration of rare earth elements and the redistribution of Zn and O isotopes in metamorphosed phosphorite, which suggests that the varying degrees of metamorphic fluid–rock reaction during the metamorphism process significantly reduced the δ18O value of phosphate (13.7‰ ± 2.2‰ of Jinping phosphate and 7.3‰ ± 1.8‰ of Xinpu phosphate) and added light Zn isotope. These metamorphic fluids were likely generated by the metamorphic dehydration of upper slab uplift during the plate subduction in the Triassic. The negative correlation between the O (and Zn) isotope compositions and REY contents in P-rich rocks suggests that REY were likely re-enriched during the recrystallization process of apatite. Preferential migration of LREE led to significant LREE and MREE enrichment in Haizhou metamorphosed P-rich rocks compared to Weng'an unmetamorphosed phosphorites.

Emergence of the 2.1 Ga Francevillian biota was preceded by unprecedented hydrothermally driven seawater eutrophication

Recently, two independent studies suggest that the emergence of putative fossilized macro-eukaryotes in the Paleoproterozoic Francevillian Basin, ~2.1 billion years ago, may be related to a rise in seawater Zn bioavailability. This explanation is reliant on their extraordinary high Zn content and association with light Zn isotopes characteristic of eukaryotic enrichment. However, the trigger and origin of rising seawater Zn supply to the basin remains unknown. This study unravels a transient episode of intense submarine hydrothermal activity that triggered the weathering of a nutrient-rich oceanic crust reservoir, related to the collision of the Congo-São Francisco cratons during the Eburnean-Transamazonian orogeny, as the source of abundant seawater dissolved Zn, together with a suite of essential trace metals and phosphate to the continental margin waters. Surprisingly, the initiation of hydrothermal weathering coincided with the rapid onset of a rare Paleoproterozoic seawater eutrophication event. This transition is marked by basin-wide redox stratification, high sediment loading with organic carbon (Corg) and nitrogen, elevated C/N ratios, a steep negative Corg and positive bulk N isotope excursion, positive Ce anomalies, and low Mn/Fe ratios. Importantly, the transient eutrophication event ended with a reversal to lower seawater phosphate levels that coincided with rapid seawater ventilation and the appearance of macrofossil bearing sediments in Franceville. We suggest that these unexpected, localized conditions, set the stage for the emergence of the Francevillian biota.

Zinc Stable Isotope Fractionation Mechanisms during Adsorption on and Substitution in Iron (Hydr)oxides.

The Zn isotope fingerprint is widely used as a proxy of various environmental geochemical processes, so it is crucial to determine which are the mechanisms responsible for isotopic fractionation. Iron (Fe) (hydr)oxides greatly control the cycling and fate and thus isotope fractionation factors of Zn in terrestrial environments. Here, Zn isotope fractionation and related mechanisms during adsorption on and substitution in three FeOOH polymorphs are explored. Results demonstrate that heavy Zn isotopes are preferentially enriched onto solids, with almost similar isotopic offsets (Δ66/64Znsolid-solution = 0.25-0.36‰) for goethite, lepidocrocite, and feroxyhyte. This is consistent with the same average Zn-O bond lengths for adsorbed Zn on these solids as revealed by Zn K-edge X-ray absorption fine structure spectroscopy. In contrast, at an initial Zn/Fe molar ratio of 0.02, incorporation of Zn into goethite and lepidocrocite by substituting for lattice Fe preferentially sequesters light Zn isotopes with Δ66/64Znsubstituted-stocksolution of -1.52 ± 0.09‰ and -1.18 ± 0.15‰, while Zn-substituted feroxyhyte (0.06 ± 0.11‰) indicates almost no isotope fractionation. This is closely related to the different crystal nucleation and growth rates during the Zn-doped FeOOH formation processes. These results provide direct experimental evidence of incorporation of isotopically light Zn into Fe (hydr)oxides and improve our understanding of Zn isotope fractionation mechanisms during mineral-solution interface processes.

Zn isotope signatures in soil Fe[sbnd]Mn nodules with karst high geochemical background

Zn isotope has the potential to be used as an environmental tracer, due to its role in fingerprinting specific sources and processes. However, few studies have focused on Zn isotope system in terrestrial ferromanganese (FeMn) nodules, which is important on understanding the behaviors of Zn in soils. In this study, we analyse the isotopic composition in soil FeMn nodules and surrounding materials from a typical karst region in Guangxi Province, southwestern China and use advanced synchrotron-based methods to characterize Zn speciation. The Zn isotope compositions of the FeMn nodules range from 0.09 to 0.66 ‰, with an average value of 0.24 ‰. Pb isotope fingerprinting reveals that the major material sources contributing to the FeMn nodules are the surrounding soil (δ66Zn: ~0.36 ‰) and partly weathered carbonate bedrock (δ66Zn: ~0.58 ‰), which contain heavier Zn isotopes than the nodules. Synchrotron-based X-ray fluorescence (μ-SXRF) shows that Zn is well correlated with both Fe and Mn. X-ray absorption near edge spectroscopy (XANES) measurements reveal that Zn is associated with both goethite and birnessite phases, with goethite-sorbed Zn accounting for ~76 % of the total Zn and birnessite-sorbed Zn accounting for ~24 %. By combining these new results, the isotopically light Zn in the FeMn nodules compared to their sources can be explained by equilibrium sorption of Zn on goethite and birnessite, during which light Zn is preferentially sorbed. Our study provides important new data on Zn isotope compositions in terrestrial soil FeMn nodules and constrains associated mechanisms, and have implications for using Zn isotopes as environmental tracers.

The Importance of Reversible Scavenging for the Marine Zn Cycle Evidenced by the Distribution of Zinc and Its Isotopes in the Pacific Ocean

AbstractThe North Pacific has played an important role in ongoing discussions on the origin of the global correlation between oceanic dissolved Zn and Si, while data in the North Pacific have remained sparse. Here, we present dissolved Zn and δ66Zn data from the US GEOTRACES GP15 meridional transect along 152°W from Alaska to the South Pacific. In the south (<20°N) Zn and Si exhibit a tight linear correlation reflecting strong Southern Ocean influence, while in the north (>20°N) an excess of Zn relative to Si in upper and intermediate waters is due to regeneration of Zn together with PO4. Using a mechanistic model, we show that reversible scavenging is required as an additional process transferring Zn from the upper to the deep ocean, explaining the deep Zn maximum below the PO4 maximum. This mechanism applied for reversible scavenging also provides an explanation for the observed isotope distribution: (a) fractionation during ligand binding and subsequent removal of residual heavy Zn in the upper ocean, drives the upper ocean toward lower δ66Zn, while (b) release of heavy Zn then coincides with the PO4 maximum where carrier particles regenerate, causing a mid‐depth δ66Zn maximum. In the upper ocean, seasonal physical stratification is an additional important process influencing shallow δ66Zn signals. At the global scale, this mechanism invoking fractionation during ligand binding coupled with reversible scavenging offers a global explanation for isotopically light Zn at shallow depths and corresponding elevated mid‐depth δ66Zn signals, seen dominantly in ocean regions away from strong Southern Ocean control.

Zinc Supply Affects Cadmium Uptake and Translocation in the Hyperaccumulator Sedum Plumbizincicola as Evidenced by Isotope Fractionation.

This study employs stable isotope analysis to investigate the mechanisms of cadmium (Cd) and zinc (Zn) interaction in the metal hyperaccumulating plant species Sedum plumbizincicola. To this end, the Cd and Zn isotope compositions of root, stem, leaf, and xylem sap samples were determined during metal uptake and translocation at different Cd and Zn concentrations. The enrichment of light isotopes of both elements in plants during uptake was less pronounced at low metal supply levels, likely reflecting the switch from a low-affinity to a high-affinity transport system at lower levels of external metal supply. The lower δ114/110Cd values of xylem sap when treated with a metabolic inhibitor decreasing the active Cd uptake further supports the preference of heavier Cd isotopes during high-affinity transport. The Δ66Znplant-initialsolution or Δ66Znplant-finalsolution values were similar at different Cd concentrations, indicating negligible interaction of Cd in the Zn uptake process. However, decreasing Zn supply levels significantly increased the enrichment of light Cd isotopes in plants (Δ114/110Cd = -0.08‰) in low-Cd treatments but reduced the enrichment of light Cd isotopes in plants (Δ114/110Cd = 0.08‰) under high Cd conditions. A systematic enrichment of heavy Cd and light Zn isotopes was found in root-to-shoot translocation of the metals. The Cd concentrations of the growth solutions thereby had no significant impact on Zn isotope fractionation during root-to-shoot translocation. However, the Δ114/110Cdtranslocation values hint at possible competition between Cd and Zn for transporters during root-to-shoot transfer and this may impact the transport pathway of Cd. The stable isotope data demonstrate that the interactions between the two metals influenced the uptake and transport mechanisms of Cd in S. plumbizincicola but had little effect on those of Zn.

An investigation of zinc isotope fractionation in cacao (Theobroma cacao L.) and comparison of zinc and cadmium isotope compositions in hydroponic plant systems under high cadmium stress

This study aims to establish whether zinc (Zn) and cadmium (Cd) share similar physiological mechanisms for uptake and translocation in cacao plants (Theobroma cacao L.). Multiple-collector ICP-MS was used to determine the Zn stable isotope compositions in the roots, stems and leaves of 19 diverse cacao genotypes grown in hydroponics with 20 µmol L−1 CdCl2. Additional plants of one genotype were grown in hydroponic solutions containing lower Cd concentrations (0 and 5 µmol L−1 added CdCl2). Regardless of the Cd concentration used in the exposures, the Zn stable isotope compositions show the same systematic patterns in plant organs, with δ66Znroot > δ66Znstem > δ66Znleaf (δ66Zn denotes relative differences in 66Zn/64Zn ratios in parts per thousand). The mean Zn stable isotope fractionation between the plants and the hydroponic solutions was ε66Znuptake = –1.15 ± 0.36‰ (2SD), indicating preferential uptake of isotopically light Zn by plants from the hydroponic solution. The mean stable isotope fractionation factor associated with translocation of Zn from roots to shoots, ε66Znseq-mob = + 0.52 ± 0.36‰ (2SD), shows that isotopically heavy Zn is preferentially sequestered in the cacao roots, whilst isotopically light Zn is mobilised to the leaves. A comparison with the Cd stable isotope compositions of the same plants shows that both isotopically light Zn and Cd are preferentially taken up by cacao plants. In contrast to Zn, however, the cacao roots retain isotopically light Cd and transfer isotopically heavy Cd to the leaves.

Can Zn isotopes in sediments record past eutrophication of freshwater lakes? A pilot study at Lake Baldegg (Switzerland)

In this study, the speciation and isotopic composition of Zn were traced across the sediments of a freshwater lake that experienced one hundred years of strong eutrophication, in order to assess the potential of sedimentary Zn isotopes to record such an environmental disturbance. The results indicate that the sedimentary Zn isotope signal varied with the change from pre-eutrophic to eutrophic conditions in the investigated lake. The average δ66ZnJMC value of the dominantly allochthonous lithogenic sediments deposited during the pre-eutrophic period was +0.27‰ ±0.05‰ (i.e. similar to the δ66ZnJMC value of +0.28‰ ±0.05‰ proposed for Bulk Silicate Earth), while enhanced autochthonous biochemical sedimentation during the eutrophic period resulted in significantly lower δ66ZnJMC values down to +0.04‰ ±0.06‰. Synchrotron-based X-ray absorption spectroscopy data revealed a concomitant change in Zn speciation from a dominant fraction of Zn in clay minerals during the pre-eutrophic period to a major fraction of Zn in ZnS during the eutrophic period. A linear regression relating the sedimentary Zn isotope signal to the fraction of Zn in ZnS indicated δ66ZnJMC values of +0.27‰ ±0.06‰ and 0.00‰ ±0.08‰ for Zn in clay minerals and in ZnS, respectively. The enrichment of light Zn in ZnS in the eutrophic sediments is tentatively attributed to enhanced biological uptake of light Zn in the water column, which resulted in an enhanced flux of organic-bound Zn towards the sediments and further transformation of organic Zn into ZnS upon biomass mineralization during early diagenesis. This hypothesis is in agreement with the fractionation towards lighter Zn reported for both biological uptake of Zn and ZnS precipitation. The results of this study emphasize the potential of sedimentary Zn isotopes to register past eutrophic periods in freshwater lakes, and thus to serve as a probe of paleo-environmental conditions and/or past land use at the catchment scale.

Zinc isotopic evidence for enhanced continental weathering and organic carbon burial during the late Cambrian SPICE event

The globally recognized late Cambrian Steptoean Positive Carbon Isotope Excursion (SPICE; ca. 497–494 Ma) is one of the most prominent carbon isotope excursions during the Phanerozoic eon, yet its primary cause remains debatable. To provide new insights into this issue, here we investigate zinc stable isotopes (expressed as δ66Zn) on two coeval carbonate successions spanning the Miaolingian-Furongian transition in North China. Similar stratigraphic δ66Zn trends are observed in the two sections, indicating a widespread perturbation to the marine Zn cycling. An abrupt decline of δ66Zn values by up to 0.4‰ and a concomitant increase of Zn/Ca ratios are observed at the pre-SPICE to the early SPICE interval, which can be best explained by the enhanced influx of isotopically light Zn from continental weathering. Enhanced continental weathering increased the inputs of terrestrial-sourced nutrients to the oceans, promoting the primary productivity and burial fluxes of organically bound Zn. At the middle stage of the SPICE, the coupled positive shifts of δ66Zn and δ13Ccarb indicate a phase of marine Zn cycling fluctuations driven by massive organic matter burial. Enhanced sequestration of organic carbon caused a rise in atmospheric oxygen levels, favoring the remobilization of previously buried Zn at shallower marine settings. With the development of muted continental weathering during the falling limb of the SPICE, the return of δ66Zn to initial background values marks the reestablishment of the stable seawater chemistry condition. Our results constrain the important roles of enhanced continental weathering and organic matter burial in micronutrient fluctuations during the SPICE, which were most likely the primary trigger for the SPICE.

Hydrothermal remobilization of subseafloor sulfide mineralization along mid-ocean ridges contributes to the global oceanic zinc isotopic mass balance

Hydrothermal activity on mid-ocean ridges is an important mechanism for the delivery of Zn from the mantle to the surface environment. Zinc isotopic fractionation during hydrothermal activity is mainly controlled by the precipitation of Zn-bearing sulfide minerals, in which isotopically light Zn is preferentially retained in solid phases rather than in solution during mineral precipitation. Thus, seafloor hydrothermal activity is expected to supply isotopically heavy Zn to the ocean. Here, we studied sulfide-rich samples from the Duanqiao-1 hydrothermal field, located on the Southwest Indian Ridge. We report that, at the hand-specimen scale, late-stage conduit sulfide material has lower δ66Zn values (−0.05 ± 0.15 ‰; n = 19) than early-stage material (+0.13 ± 0.15 ‰; n = 10). These lower values correlate with enrichments in Pb, As, Cd, and Ag, and elevated δ34S values. We attribute the low δ66Zn values to the remobilization of earlier sub-seafloor Zn-rich mineralization. Based on endmember mass balance calculations, and an assumption of a fractionation factor (αZnS-Sol.) of about 0.9997 between sphalerite and its parent solution, the remobilized Zn was found consist of about 1/3 to 2/3 of the total Zn in the fluid that formed the conduit samples. Our study suggests that late-stage subsurface hydrothermal remobilization may release isotopically-light Zn to the ocean, and that this process may be common along mid-ocean ridges, thus increasing the size of the previously identified isotopically light Zn sink in the ocean.

Isotopic evidence for changes in the mercury and zinc cycles during Oceanic Anoxic Event 2 in the northwestern Tethys, Austria

The Cenomanian-Turonian Oceanic Anoxic Event 2 (OAE 2, ca. 94 Ma) was one of the most extreme carbon cycle and climatic perturbations of the Phanerozoic Eon. Widespread deposition of organic-rich shales during OAE 2 has been attributed to a rapid rise in atmospheric CO2, global heating, and marine anoxia triggered by intense large igneous province (LIP) volcanism. Here, we present new Hg and Zn elemental and isotopic analyses from samples spanning OAE 2 in a hemipelagic section from Rehkogelgraben, Austria, which was part of the north-western Tethys. We compare our data to existing records from a range of sites to constrain the relative timing, magnitude and geographic extent of the perturbation. We find a prominent Hg concentration peak and an overall positive Δ199Hg excursion, with no correlation between Hg content and organic matter (OM), Mn-Fe-oxyhydroxides, and/or clay minerals. We interpret this to indicate a terrestrial volcanic origin of Hg. The Hg excursion is coincident with an osmium (Os) isotope excursion, and together, this supports a global period of intense LIP volcanism. The δ66Zn record from the Rehkogelgraben section decreases abruptly by ~0.5‰ prior to the onset of OAE 2, a change recorded consistently among all reference sections. Combined with the Hg data, we interpret this to result from isotopically light Zn sourced from LIP activity. However, the second negative excursion in δ66Zn during the Plenus Cold Event (PCE), which is recorded in the proto-North Atlantic and adjacent areas and has been attributed to Zn released from OM during re‑oxygenation, is not recorded in this section. We suggest that the cool, oxygenated deep water mass did not invade the Penninic Ocean in the northwestern Tethys. Alternatively, this excursion could be missing in our section due to the presence of carbonate-free sediments during the PCE. After the PCE, the positive excursion in δ66Zn recorded in all sections reveals a recovery of the atmosphere-ocean system. Our findings highlight the significance of spatial and temporal variations in Hg and Zn isotopes during OAE 2.

Light Zn and Cu isotope compositions recorded in ferromanganese crusts during the Cenozoic as evidence for hydrothermal inputs in South Pacific deep seawater

This study presents a high-resolution record of Cu and Zn isotopes in four Fe-Mn crusts from the North and South Pacific oceans. North Pacific crusts were collected on the Apuupuu seamount south of the Hawaiian archipelago and South Pacific crusts were recovered near Rurutu Island in the Tahiti archipelago. Major and trace element compositions suggest that Cu and Zn in these crusts is of hydrogenous origin, i.e., precipitated from seawater, and they may therefore mirror deep seawater metal isotope. We show that Cu and Zn display different isotopic patterns between the North and the South Pacific Oceans but show similar temporal evolution within each geographical area. Copper and Zn isotope composition of both North Pacific crusts vary between 0.57 ‰ to 0.73 ‰ for δ65/63CuNIST976 and 0.97 ‰ to 1.25 ‰ for δ66/64ZnJMC-Lyon. In contrast, South Pacific crusts show resolvable temporal variations, with Cu and Zn isotopic ratios increasing sharply over the last ∼ 6 Ma from 0.16 ‰ to 0.51 ‰ and 0.67 ‰ to 1.09 ‰ respectively. Notably, we observed a positive correlation between δ65/63CuNIST976 and δ66/64ZnJMC-Lyon values in Fe-Mn crusts from the South Pacific. The correlation suggests mixing between two components in Fe-Mn crusts, a hydrothermal component with δ65/63CuNIST976 ∼ 0.2 ‰ and δ66/64ZnJMC-Lyon ∼ 0.7 ‰, and a Pacific deep seawater component with δ65/63CuNIST976 ∼ 0.7 ‰ and δ66/64ZnJMC-Lyon ∼ 1.2 ‰. These values are fractionated from modern dissolved Cu and Zn by a factor of −0.3 ‰ and 0.5 ‰ respectively. We suggest that the deep Southern Pacific Ocean received sustained hydrothermal input during the last 6 Ma, which was recorded in the Cu and Zn isotope composition of Fe-Mn crusts precipitated thousands of kilometers away. Our study highlights that hydrothermal venting may be a significant source of Cu and Zn in the deep oceans despite their extensive precipitation within hydrothermal vent fields. We show that this source could be persistent through time, and thus, it could have a significant impact on the biogeochemical cycling of Cu and Zn in seawater which would ultimately be recorded by Fe-Mn crusts.

Zn isotope fractionation in laterites from Yunnan province, southwest China: Implications for the Zn cycles and its environmental impacts in (sub-) tropics

The weathering and development of laterites can influence trace element cycling in (sub-) tropics. Zinc (Zn) is a ubiquitous trace metal that involves both abiotic and biotic processes in soils. To explore Zn behavior in laterites, Zn cycling in (sub-) tropics, and the environmental impacts, Zn isotope systematics were presented for two laterite profiles from Yunnan province, southwest China. The laterite samples exhibit the δ66Zn of 0.02 ‰–0.56 ‰, indicating a light shift of Zn isotope ratios (Δ66Znlaterite-parent rock = −0.47 ‰–0.07 ‰) relative to bulk parent granite. This observation is attributed to the preferential preservation of light Zn isotopes on the surface of secondary Fe oxides. As a result, laterites are likely to control the instantaneous riverine δ66Zn in (sub-) tropical regions heavier than unweathered rocks. The isotopic signature of different vegetation covered soils show that shrub-covered soils are stronger leached (average τZn = −0.61) and have a smaller Δ66Znlaterite-parent rock (=−0.15 ‰), relative to forest-covered soils (=−0.20 ‰). Due to the strong loss of Zn (average τZn = −0.61 to −0.12) and large amounts of low-bioavailable Zn preserved in oxides, the micronutrient supplies for plant growth are difficult to maintain and need more fertilization. This study is helpful for a better understanding of global Zn cycling and the management of micronutrients in (sub-) tropical soil-plant systems.

Resolving impact volatilization and condensation from target rock mixing and hydrothermal overprinting within the Chicxulub impact structure

This work presents isotopic data for the non-traditional isotope systems Fe, Cu, and Zn on a set of Chicxulub impactites and target lithologies with the aim of better documenting the dynamic processes taking place during hypervelocity impact events, as well as those affecting impact structures during the post-impact phase. The focus lies on material from the recent IODP-ICDP Expedition 364 Hole M0077A drill core obtained from the offshore Chicxulub peak ring. Two ejecta blanket samples from the UNAM 5 and 7 cores were used to compare the crater lithologies with those outside of the impact structure. The datasets of bulk Fe, Cu, and Zn isotope ratios are coupled with petrographic observations and bulk major and trace element compositions to disentangle equilibrium isotope fractionation effects from kinetic processes. The observed Fe and Cu isotopic signatures, with δ56/54Fe ranging from −0.95‰ to 0.58‰ and δ65/63Cu from −0.73‰ to 0.14‰, mostly reflect felsic, mafic, and carbonate target lithology mixing and secondary sulfide mineral formation, the latter associated to the extensive and long-lived (>105 years) hydrothermal system within Chicxulub structure. On the other hand, the stable Zn isotope ratios provide evidence for volatility-governed isotopic fractionation. The heavier Zn isotopic compositions observed for the uppermost part of the impactite sequence and a metamorphic clast (δ66/64Zn of up to 0.80‰ and 0.87‰, respectively) relative to most basement lithologies and impact melt rock units indicate partial vaporization of Zn, comparable to what has been observed for Cretaceous-Paleogene boundary layer sediments around the world, as well as for tektites from various strewn fields. In contrast to previous work, our data indicate that an isotopically light Zn reservoir (δ66/64Zn down to −0.49‰), of which the existence has previously been suggested based on mass balance considerations, may reside within the upper impact melt rock (UIM) unit. This observation is restricted to a few UIM samples only and cannot be extended to other target or impact melt rock units. Light isotopic signatures of moderately volatile elements in tektites and microtektites have previously been linked to (back-)condensation under distinct kinetic regimes. Although some of the signatures observed may have been partially overprinted during post-impact processes, our bulk data confirm impact volatilization and condensation of Zn, which may be even more pronounced at the microscale, with variable degrees of mixing between isotopically distinct reservoirs, not only at proximal to distal ejecta sites, but also within the lithologies associated with the Chicxulub impact crater.

Zinc isotopic systematics of the Mt. Baekdu and Jeju Island intraplate basalts in Korea, and implications for mantle source lithologies

Zinc isotope data for late Cenozoic intraplate basalts from Mt. Baekdu (North Korea) and Jeju Island (South Korea) were determined to characterize their mantle source lithologies. The average δ66Zn values (relative to JMC-Lyon) are 0.34‰ ± 0.08‰ (2SD; N = 13) for the Mt. Baekdu basalts and 0.37‰ ± 0.08‰ (2SD; N = 12) for the Jeju basalts, which are higher than values for refractory peridotites (0.18‰ ± 0.06‰) from the lithospheric mantle beneath the North China Craton. Given that Zn isotopic fractionation is limited during lithospheric processes, such as crustal contamination, magmatic differentiation, degassing, or thermal/chemical diffusion, the elevated δ66Zn values indicate the incorporation of recycled carbonates into the sub-lithospheric mantle source. Plots of Zn/Fe versus δ66Zn or δ26Mg for the Korean basalts can be explained by mixing of at least three source lithologies: (1) carbonated peridotite; (2) recycled siliciclastic sediments; and (3) eclogite. Strontium–Nd–Pb isotopic systematics show that the mantle source follows the DMM-EM1 array for the Mt. Baekdu basalts and a DMM-EM2 array for the Jeju basalts. Both the enriched components have isotopically light Zn and heavy Mg, which are typical of recycled siliciclastic sediments. Combined with the Pb isotope and trace element data, the Zn isotopic compositions suggest that the EM1 component could be ancient K-hollandite-bearing siliciclastic sediments and the EM2 component could be recently recycled clay-rich pelagic sediments.

Investigations on Zinc Isotope Fractionation in Breast Cancer Tissue Using in vitro Cell Culture Uptake-Efflux Experiments.

Zinc (Zn) accumulates in breast cancer tumors compared to adjacent healthy tissue. Clinical samples of breast cancer tissue show light Zn isotopic compositions (δ66Zn) relative to healthy tissue. The underlying mechanisms causing such effects are unknown. To investigate if the isotopic discrimination observed for in vivo breast cancer tissue samples can be reproduced in vitro, we report isotopic data for Zn uptake-efflux experiments using a human breast cancer cell line. MDA-MB-231 cell line was used as a model for triple receptor negative breast cancer. We determined Zn isotope fractionation for Zn cell uptake (Δ66Znuptake) and cell efflux (Δ66Znefflux) using a drip-flow reactor to enable comparison with the in vivo environment. The MDA-MB-231 cell line analyses show Zn isotopic fractionations in an opposite direction to those observed for in vivo breast cancer tissue. Uptake of isotopically heavy Zn (Δ66Znuptake = +0.23 ± 0.05‰) is consistent with transport via Zn transporters (ZIPs), which have histidine-rich binding sites. Zinc excreted during efflux is isotopically lighter than Zn taken up by the cells (Δ66Znefflux = −0.35 ± 0.06‰). The difference in Zn isotope fractionation observed between in vitro MDA-MB-231 cell line experiments and in vivo breast tissues might be due to differences in Zn transporter levels or intercellular Zn storage (endoplasmic reticulum and/or Zn specific vesicles); stromal cells, such as fibroblasts and immune cells. Although, additional experiments using other human breast cancer cell lines (e.g., MCF-7, BT-20) with varying Zn protein characteristics are required, the results highlight differences between in vitro and in vivo Zn isotope fractionation.